The present invention relates to a sheet unloading apparatus for a rotary drum rotatable in one direction along with a sheet which is wound around a peripheral surface of the rotary drum.
Jpn. Pat. Appln. KOKAI Publication No. 10-138520 discloses a rotary drum ink-jet printer which can print a large number of pages in a short period of time. The rotary drum ink-jet printer comprises a rotary drum rotatable in one direction and a print head for printing an image on a paper sheet wound around the rotary drum and rotated along with the rotary drum. The print head has a plurality of ink-jet nozzles arranged in a line across a paper sheet in the axial direction of the rotary drum. The print head jets ink through the ink-jet nozzles to the paper sheet which moves relative to the print head as the rotary drum rotates. This structure allows high-speed print, since it does not require that the print head move across the paper sheet as in the conventional serial ink-jet printer.
In general, a paper sheet is held on the rotary drum by a combination of vacuum attraction, electrostatic attraction, mechanical engagement, etc. Jpn. Pat. Appln. KOKAI Publication No. 10-193719 discloses a technique of mechanically engaging the front and rear ends of an externally supplied paper sheet on the peripheral surface of a rotary drum to prevent positional deviation of the paper sheet. According to this technique, engagement grooves are formed in the peripheral surface of the rotary drum at positions corresponding to the front and rear ends of the paper sheet. The front and rear ends of the paper sheet are engaged in the engagement grooves by clamping claws. The technique of engaging the rear end of a paper sheet is also disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-329375.
In the rotary drum ink-jet printer, at the time of unloading the paper sheet, the front end of a paper sheet is separated from the drum by a separation pawl which is brought into contact with the rotary drum, and then guided to a pair of pinch rollers. These pinch rollers are disposed near the rotary drum to pinch the front end of the paper sheet in a state where the rear end thereof is engaged in the engagement groove, and rotate in one direction at the same circumferential speed as the circumferential speed of the rotary drum to feed out the pinched paper sheet.
However, since the shapes of the engagement grooves and the rigidity of the paper sheet are determined to gain sufficient sheet holding force, it is possible that the rear end of the paper sheet may not be removed from the engagement groove by the sheet pulling force of the pinch rollers. If the rotary drum is further rotated together with the paper sheet whose the rear end is engaged in the engagement groove, the paper sheet will be pulled out of the pinch rollers in the reverse direction and wound around the rotary drum inside out, causing a paper jam. This problem can be overcome by increasing the rotation speed of the pinch rollers. However, since the paper sheet is easily waved by shock resulting from a difference in circumferential speed between the rotary drum and the pinch rollers, it is necessary to change the circumferential speed of the pinch rollers after the rear end of the paper sheet passes by the print head for printing an image on the paper sheet. This change of speed requires a complicated structure for a pinch roller driving circuit, resulting in an increase in cost, restriction on the layout and an increase in power consumption. Further, it is not easy to determine the optimal speed of the pinch rollers, which provides sheet pulling force slightly greater than the sheet holding force on the rotary drum. If the shape of the engagement grooves, such as the depth or the slope angle, or the rigidity of the paper sheet is to be changed, the optimal speed must be reset in accordance with such a change.